Multi-cylinder engine blocks have long been produced by casting processes and then machined and assembled into reciprocating piston, internal combustion engines for automotive vehicles and for other power requirements. The cast engine blocks including the cylinder internal diameters or surfaces (sometimes called “cylinder bores”) are machined for precision fit with other engine parts including a cylinder head and the pistons (with their piston rings) which reciprocate in high speed contact with the cylinder surfaces in an operating engine. Molds for such castings with internal round cylindrical surfaces have been made of different materials, including sand molds with sand cores for defining internal cylindrical surfaces and permanent metal molds with retractable core pieces (mandrels) for shaping cylindrical surfaces. Such multi-cylinder castings have long been made of cast iron and, in more recent decades, of aluminum alloys, and may be made of magnesium alloys in the future.
Die cast aluminum alloy cylinder blocks for vehicular internal combustion engines, especially gasoline-fueled engines, have been produced for many years. Typically, the cylinder blocks are cast using a silicon-containing aluminum alloy composition that provides suitable fluidity in its molten state for forming the intricate shapes of cylinder blocks with their closely spaced cylinder bores, coolant passages, and other engine block features. But the aluminum alloy compositions have not displayed enough hardness and wear resistance on cylinder surfaces to resist damage by the pistons and rings reciprocating in sliding engagement with the cylinder surfaces in an operating engine. So wear-resistant iron cylinder liners (or of other wear resistant materials) have been placed in the casting mold and the aluminum alloy cast around the liners as the cylinder block is molded. The solidified aluminum composition forms most of the engine block while the cast-in-place liners are anchored to the surrounding aluminum and provide hard cylinder wall surfaces.
Now at least one aluminum alloy composition has been developed that provides both fluidity for casting of engine blocks and wear resistance against piston/ring wear. These alloys may be cast in sand molds with sand cores to make multi-cylinder engine blocks without special wear resistant liners. But for higher production volumes it is desired to use high pressure die casting machines to mold such aluminum alloys. However, when some molten aluminum alloys are forced into direct contact with metal mandrels under high pressure the aluminum composition adheres to the mandrel surfaces. Further, as the material solidifies it shrinks tightly against the mandrels and it is difficult to extract the casting tools from the solidified cylinder block without damaging expensive tools and/or the internal cylindrical surfaces of the casting.
It is an object of this invention to provide a method for high pressure die casting of molten aluminum-base alloys against metal casting tool surfaces (often ferrous metal surfaces) that avoids sticking of the aluminum materials to mandrels or other tool surfaces. The method may also be useful in die casting magnesium alloys or other materials, especially in casting arrangements when the metal shrinks inwardly against the tool surface and otherwise adheres to it.